In the present monograph, an attempt is made to give an account of the fundamentals of the theory of luminescence or, more precisely, the theory of photo-luminescence. The first three chapters are concerned with classical emissiontheory, and the quantum mechanics and quantum-electro­ dynamics which are necessary for the understanding of the physical processes leading to luminescence.The next two chapters discuss the general principles of the theory of absorption and luminescence without reference to any specific models of matter. These chapters are devo­ ted to a detailed study of the optical properties of the har­ monic oscillator and of systems of particles with two, three or more energy levels.Much of the material given in this book is based on the original work carried out at the Institute of Physics of the Academy of Sciences Byelorussian SSR. In particular, a detailed description is given of the effect of the thermal emission background, the properties of negative radiation fluxes and negative luminescence. Non-linear optical phen­ omena which arise in the interaction of matter with high, and occasionally with ordinary, fluxes of radiation are systematically investigated. They include departures from Bouguer’s law, depolarisation, induced dichroism and amplification and generation of radiation in media with negative absorption coefficients.